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CONTRAST:
Convective Transport of Active Species in the Tropics

Summary

The Convective Transport of Active Species in the Tropics (CONTRAST) experiment is designed to measure the chemistry and transport of reactive chemical species into the tropical Tropopause Transition Layer (TTL) over the Western Pacific warm pool area. During the boreal winter season, tropospheric air masses are preferentially transported into the lower stratosphere in this region. Thus, the sources, chemistry, and transport of trace gases and their degradation products in the region can substantially impact the chemistry of the lower stratosphere. Considerable attention is being given to the role of tropical convection on the delivery of reactive gases to the TTL. For example, uncertainties in the abundance of reactive halogen species and the fate of short lived organic halogen compounds, particularly bromocarbons, in the tropical upper troposphere leads to significant uncertainties in the photochemistry of stratospheric ozone, especially in the lowermost stratosphere. The CONTRAST mission will provide new measurements in this data sparse region to determine the influence of convection on the reactive chemistry of the tropical upper troposphere, to evaluate the role of different transport pathways in shaping the chemical composition, and to define the chemical budgets for air entering the stratosphere in this region. The research proposed here is to measure a wide range of organic trace gases collected from the HAIS Advanced Whole Air Sampler (AWAS) on the NCAR GV aircraft. Up to 60 samples will be collected on each flight in either autonomous or on-command sampling mode. The trace gases to be targeted include a range of organic molecules, including a both short-lived and longlived halocarbon compounds, as well as non-methane hydrocarbons, organic nitrates, and selected sulfur species. The gases targeted for this study will provide information to diagnose halogen transformations and budget of the TTL, to estimate relative age of air in the UT/LS, and to distinguish air mass sources in the region. Trace organic gas data analysis will be coordinated with similar measurements from the TOGA and other instruments on the GV and with two collaborating research missions (ATTREX and CAST). The combined data will provide a unique characterization of tropical atmospheric chemical composition from the surface through the TTL and into the lower tropical stratosphere to 20 km altitude.